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Restore localhost fallback in bin/dig/dighost.c

Browse source 
In BIND 9.11 and earlier, dig and similar tools used liblwres for
parsing /etc/resolv.conf.  After getting a list of servers from
liblwres, a tool would check the address family of each server found and
reject those unusable.  When the resulting list of usable servers was
empty, localhost addresses were queried as a fallback.

When liblwres was removed in BIND 9.12, dig and similar tools were
updated to parse /etc/resolv.conf using libirs instead.  As part of that
process, the localhost fallback was removed from bin/dig/dighost.c since
the localhost fallback built into libirs was deemed to be sufficient.
However, libirs only falls back to localhost if it does not find any
name servers at all; if it does find any valid nameserver entry in
/etc/resolv.conf, it just returns it to the caller because it is
oblivious to whether the caller supports IPv4 and/or IPv6 or not.  The
code in bin/dig/dighost.c subsequently filters the returned list of
servers in get_server_list() according to the requested address family
restrictions.  This may result in none of the addresses returned by
libirs being usable, in which case a tool will attempt to work with an
empty server list, causing a hang and subsequently a crash upon user
interruption.

Restore the localhost fallback in bin/dig/dighost.c to prevent the
aforementioned hangs and crashes and ensure recent BIND versions behave
identically to the older ones in the circumstances described above.
This commit is contained in:
Michał Kępień 2018-11-13 14:31:18 +01:00
parent 0507658884
commit 18758392da
1 changed files with 23 additions and 0 deletions
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23
bin/dig/dighost.c
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@ -1227,6 +1227,19 @@ create_search_list(irs_resconf_t *resconf) {
}
}
/*%
* Append 'addr' to the list of servers to be queried. This function is only
* called when no server addresses are explicitly specified and either libirs
* returns an empty list of servers to use or none of the addresses returned by
* libirs are usable due to the specified address family restrictions.
*/
static void
add_fallback_nameserver(const char *addr) {
dig_server_t *server = make_server(addr, addr);
ISC_LINK_INIT(server, link);
ISC_LIST_APPEND(server_list, server, link);
}
/*%
* Setup the system as a whole, reading key information and resolv.conf
* settings.
@ -1272,6 +1285,16 @@ setup_system(bool ipv4only, bool ipv6only) {
get_server_list(resconf);
}
/* If we don't find a nameserver fall back to localhost */
if (ISC_LIST_EMPTY(server_list)) {
if (have_ipv6) {
add_fallback_nameserver("::1");
}
if (have_ipv4) {
add_fallback_nameserver("127.0.0.1");
}
}
irs_resconf_destroy(&resconf);
#ifdef HAVE_SETLOCALE